This application claims the benefit of a Japanese Patent Application No.11-214180 filed Jul. 28, 1999, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a mobile communication system having a line control station and a plurality of clustered base stations, a base station for such a mobile communication system, a cluster control station, a line control station, and a mobile station.
2. Description of the Related Art
FIG. 1 is a diagram for explaining an example of a conventional mobile communication system. The mobile communication system shown in FIG. 1 includes a mobile station (MS) 10, base stations (BSs) 111 through 117, a line control station (RNC: Routing Network Controller) 12, and a mobile communication switching station (MSC: Mobile Switching Center) 13.
The mobile communication switching station 13 is a switching station designed for a mobile communication network. Lines of this mobile communication switching station 13 are connected to another mobile communication switching station or a switching station of a fixed network such as a Public Switched Telephone Network (PSTN). In the case where the mobile communication switching station 13 is a third generation type, the mobile communication switching station 13 not only supports the circuit switching functions, but also supports packet switching functions, which enables connection to the Internet.
The line control station 12 is connected to the mobile communication switching station 13, and to the plurality of base stations 111 through 117. When a communication request is generated, the line control station 12 controls the allocation of radio channels used by the base stations 111 through 117, the start and end of the communication, and the like. In addition, the line control station 12 carries out a handover control when the mobile station 10 moves among the coverage areas of the base stations 111 through 117.
The base stations 111 through 117 are connected to the line control station 12, and carry out radio communication with a plurality of mobile stations 10 under the control of the line control station 12. Various kinds of radio signal protocols (air interfaces), such as the Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA), may be employed between the base station and the mobile station.
When making a communication with one mobile station 10 in the mobile communication system shown in FIG. 1, radio signals are basically exchanged directly (peer-to-peer) between the mobile station 10 and the base station which is located near the mobile station 10. In the particular case shown in FIG. 1, the base station 112 is close to the one mobile station 10 which is shown. Accordingly, the signals from the mobile station 10 are sent to the mobile communication switching station 13 via the base station 112 and the line control station 12. When the mobile station 10 moves from the location near the base station 112 towards a location near the base station 113 as shown in FIG. 1, the handover is made from the base station 112 to the base station 113 in order to continue the communication. In other words, the line control station 12 controls the base stations so as to change the base station which is used for the communication with the mobile station 10 from the base station 112 to the base station 113, and also switches the line to the base station from the base station 112 to the base station 113. In FIG. 1, a solid line arrow pointing in two directions indicates a normal communication, and a dotted line arrows pointing in two directions indicates a handover communication.
In the case of a microcell mobile communication system in which an area covered by one base station is small, the handover among the base stations occur frequently when the mobile station 10 carries out the communication while moving at a high speed. As a result, the load on the line control station 12 which carries out the necessary control increases considerably in this case, and there is a problem in that it is difficult to appropriately control and carry out the handover.
On the other hand, in a vicinity of a boundary between two adjacent base stations, the electric field intensity decreases and an error may be generated in the received signals. Consequently, the handover may not be carried out smoothly, and the handover may fail, thereby causing a problem particularly in the microcell mobile communication system in which the handover occurs frequently.
In order to eliminate the latter problem, the CDMA mobile communication system employs the diversity handover technique. According to the diversity handover technique, the mobile station 10 during the handover simultaneously exchanges radio signals with the present base station 112 and the next base station 113 which is to take over, so that even if the signal level exchanged between the mobile station 10 and one of the base stations 112 and 113 decreases and the signal error is generated, the signals exchanged between the mobile station 10 and the other of the base stations 112 and 113 are used for the communication to realize a smooth handover. In this case, in addition to the load caused by the handover control, the line control station 12 must bear the load of the diversity process. Hence, the load on the line control station 12 increases considerably due to the diversity process.
Furthermore, in order to more effectively obtain the diversity effects, it is necessary to transfer to the line control station 12 information related to soft decisions made on the received signals, and information related to the electric field intensity of the received signals. As a result, the amount of information exchanged between the base station and the line control station increases considerably, and there is a problem in that it is necessary to use a line having a large capacity.
The diversity handover is not limited between two base stations, and in some cases, the diversity effects are greater when the diversity handover is carried out among three or more base stations. In addition, the diversity handover can effectively improve the quality of the communication by being carried out not only during the handover, but also when the electric field intensity of the received signals decreases when the mobile station enters within a building, for example. However, the load on the line control station and the load on the line capacity between the base station and the line control station further increases in such cases, thereby making it difficult to realize such a diversity handover in the conventional mobile communication system. Therefore, there was a limit to improving the quality of the communication in the conventional mobile communication system.
Accordingly, it is a general object of the present invention to provide a novel and useful mobile communication system, a base station, a cluster control station, a line control station, and a mobile station, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide mobile communication system, a base station, a cluster control station, a line control station, and a mobile station, which can improve the quality of the communication, reduce the load on the line control station and the line between the base station and the line control station, and carry out a distributed autonomous process in the base station, so that an efficient mobile communication system can be realized.
Still another object of the present invention is to provide a mobile communication system comprising a line control station, and a plurality of base stations belonging to a plurality of clusters so that each of the clusters includes a plurality of base stations, wherein each of the clusters includes a cluster control station which is connected to the line control station via a line and controls the base stations within the cluster to which the cluster control station belongs, and each of the base stations within one cluster is connected to other base stations within the one cluster and/or the cluster control station within the one cluster via lines, so that each of the base stations within the one cluster is directly or indirectly connected to the cluster control station within the one cluster. According to the mobile communication system of the present invention, it is possible to improve the communication quality, and reduce the load on the line control station and the load on the line between the line control station and the base station. In addition, it is possible to realize an efficient mobile communication system, by the distributed autonomous process carried out by the base station.
In the mobile communication system, connections within each of the clusters may be made by high-speed cable lines or high-speed radio lines.
In the mobile communication system, the cluster control station and each of the base stations within a single cluster may be connected via a loop network or a bus network.
In the mobile communication system, Internet Protocol (IP) may used as a communication protocol on the lines within each of the clusters. In this case, it is possible to efficiently build a local area network (LAN) within the cluster.
In the mobile communication system, a plurality of the base stations may simultaneously send signals with respect to a single mobile station. In this case, a site diversity transmission becomes possible with respect to the mobile station.
In the mobile communication system, a plurality of the base stations may simultaneously receive signals sent from a single mobile station. In this case, a site diversity reception becomes possible at the base stations.
In the mobile communication system, the plurality of clusters may be mutually connected via lines. In addition, the line control station may connect corresponding cluster control stations within the plurality of clusters via the lines. In this case, the information exchanged between mobile stations can take a bypass route around the line control station, to thereby reduce the load on the line control station.
A further object of the present invention is to provide a cluster control station for controlling a plurality of base stations within one cluster to which the cluster control station belongs, in a mobile communication system which includes a line control station and a plurality of clusters, where the cluster control station comprises means for connecting the cluster control station to the line control station via a line, and means for combining signals received from one or a plurality of base stations within the one cluster and sending combined signals to the line control station. According to the cluster control station of the present invention, it is possible to realize a soft handover and a site diversity process.
The cluster control station may be connected to the plurality of base stations within the one cluster via a loop network or a bus network.
Another object of the present invention is to provide a cluster control station for controlling a plurality of base stations within one cluster to which the cluster control station belongs, and operable in a mobile communication system which includes a line control station and a plurality of clusters, where the cluster control station comprises means for connecting the cluster control station to the line control station via a line, and means for sending signals received from the line control station to a plurality of base stations within the one cluster directly or via one or a plurality of other base stations within the one cluster. According to the cluster control station of the present invention, it is possible to realize a soft handover and a site diversity process.
The cluster control station may be connected to the plurality of base stations within the one cluster via a loop network or a bus network.
Still another object of the present invention is to provide a base station provided within one cluster which includes a cluster control station and a plurality of base stations, and operable in a mobile communication system which includes a line control station and a plurality of clusters, where the base station comprises means for sending signals received from a mobile station to the cluster control station within the one cluster directly or via one or a plurality of other base stations within the one cluster. According to the base station of the present invention, it is possible to carry out a distributed autonomous process.
The base station may further comprise means for connecting to the one or the plurality of other base stations within the one cluster via a loop network or a bus network.
A further object of the present invention is to provide a base station provided within one cluster which includes a cluster control station and a plurality of base stations, and operable in a mobile communication system which includes a line control station and a plurality of clusters, where the base station comprises means for receiving signals from the cluster control station within the one cluster and sending to a mobile station information included in the signals and required by the mobile station. According to the base station of the present invention, it is possible to carry out a distributed autonomous process.
The base station may further comprise means for connecting to the one or the plurality of other base stations within the one cluster via a loop network or a bus network.
Another object of the present invention is to provide a line control station operable in a mobile communication system which includes a plurality of clusters each including a plurality of base stations, where the line control station comprises means for carrying out a handover control between first and second clusters when a mobile station moves between areas of the first and second clusters. According to the line control station of the present invention, it is possible to realize a soft handover.
In a case where each of the clusters includes a cluster control station which controls a plurality of base stations within the cluster, the means of the line control station may carry out the handover control via cluster control stations included in the first and second clusters.
Still another object of the present invention is to provide a mobile station operable in a mobile communication system which includes a plurality of clusters each including a plurality of mutually coupled base stations, where the mobile station comprises means for simultaneously receiving signals from one or a plurality of base stations within one cluster, and means for simultaneously sending signals to one or a plurality of base stations within one cluster. According to the mobile station of the present invention, it is possible to realize a site diversity process.
In a case where each of the clusters includes a cluster control station which controls the plurality of base stations within one cluster to which the cluster control station belongs, the mobile station may further comprise means for notifying the cluster control station of one or a plurality of base stations within the one cluster capable of communicating with the mobile station.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.